Recently, a method using zinc or zinc alloy plating has been widely employed as a method for inhibiting corrosion of the surface of a metal. However, plating by itself will not provide a sufficient corrosion resistance, and thus a chromic acid treatment after plating, that is, a so-called chromate treatment, has been widely employed in industry. On the other hand, it has been pointed out that hexavalent chromium harms human bodies and the environment, and, as a result, moves to regulate the use of hexavalent chromium have gained momentum. An alternative to a coating film formed with hexavalent chromium is a rust preventive coating film in which trivalent chromium is used. For example, Patent Article 1 discloses a treatment method using a mixture of trivalent chromium, a fluoride, an organic acid, an inorganic acid and a metal salt such as cobalt sulfate. However, this bath has environmental problems since a fluoride is used in the bath. Meanwhile, Patent Article 2 proposes hexavalent chromium-free rustproofing in which a phosphoric acid, a salt of a metal such as Mo, Cr3+ or Ti, and an oxidant are used. However, in this method there is still a possibility that trivalent chromium will be oxidized into hexavalent chromium, because of using a large amount of an oxidant.
Patent Article 3 proposes a chemical conversion treatment in which phosphorus, a metal such as Mo, and trivalent chromium are used but no fluoride is used. However, as a result of our confirmation test, it was found that a satisfactory corrosion resistance could not be reproduced. In addition, Patent Article 4 discloses a treatment method in which 5 to 100 g/L of trivalent chromium, nitrate, an organic acid, and a salt of a metal such as cobalt are used. Since in this method concentrations of chromium and the like are high and the treatment is carried out at an elevated temperature, this method has the advantage that a thick film, and accordingly a good corrosion resistance can be obtained, but the disadvantage that a stable corrosion resistance cannot be obtained because of difficulty in forming a stable and dense film. In addition, the method is also disadvantageous in wastewater treatment since the treatment bath contains chromium in high concentration and a large amount of an organic acid is also used therein. In addition, as to the appearance of the film, only colorless and interference-color appearance can be obtained. In this connection, as to formation of a black trivalent chromium chemical conversion coating film on zinc-nickel (Ni % in the film is 8% or more) or zinc-iron, Patent Article 5 discloses a treatment method with an aqueous acidic solution containing a phosphorus acid compound and trivalent chromium. Meanwhile, as to formation of an interference-color trivalent chromium chemical conversion coating film on zinc-nickel (Ni % in the film is 8% or more), Patent Article 6 discloses a treatment method with an aqueous acidic solution likewise containing a phosphorus compound, trivalent chromium, and additionally halate ions. However, the Ni codeposition rate of much of actually produced zinc-nickel alloy plating falls below 8%, and thus these method have practical problems in obtaining a black appearance. Meanwhile, regarding zinc-iron alloy plating, a sufficient corrosion resistance has not been provided.
The treatment solution disclosed in Patent Article 7 developed by the present inventors provide a good black appearance and a good corrosion resistance more than comparable to chromate using hexavalent chromium. In addition, the present inventors evaluate that the treatment solution in Patent Article 8 or Patent Article 9 provides a poorer corrosion resistance but a better black appearance than conventional black chromate. However, these chemical conversion treatment solutions each have a problem of having a short treatment bath life since the treatment solution provides a reduced black appearance as zinc ions become accumulated in the treatment solution by being dissolved from zinc or zinc alloy on the surface of the treated substrate through chemical conversion treatment of the zinc or zinc alloy.
Patent Article 1: Japanese Examined Patent Application Publication No. Sho 63-015991;
Patent Article 2: Japanese Patent Application Publication No. Hei 10-183364;
Patent Article 3: Japanese Patent Application Publication No. 2000-54157;
Patent Article 4: Japanese Patent Application Publication No. 2000-509434;
Patent Article 5: U.S. Pat. No. 5,415,702;
Patent Article 6: U.S. Pat. No. 5,407,749;
Patent Article 7: Japanese Patent Application Publication No. 2003-268562;
Patent Article 8: Japanese Patent Application Publication No. 2005-187925; and
Patent Article 9: Japanese Patent Application Publication No. 2005-206872.